A class for parsing in and manipulating RFC-822 message headers, with some methods geared towards standard (and not so standard) MIME fields as specified in the various Multipurpose Internet Mail Extensions RFCs (starting with RFC 2045)

Class or instance method. For convenience, you can use this to parse a header object in from EXPR, which may actually be any expression that can be sent to open() so as to return a readable filehandle. The "file" will be opened, read, and then closed:

### Create a new header by parsing in a file:
my $head = MIME::Head->from_file("/tmp/test.hdr");

Since this method can function as either a class constructor or an instance initializer, the above is exactly equivalent to:

### Create a new header by parsing in a file:
my $head = MIME::Head->new->from_file("/tmp/test.hdr");

On success, the object will be returned; on failure, the undefined value.

The OPTIONS are the same as in new(), and are passed into new() if this is invoked as a class method.

Note: This is really just a convenience front-end onto read(), provided mostly for backwards-compatibility with MIME-parser 1.0.

Instance (or class) method. This initializes a header object by reading it in from a FILEHANDLE, until the terminating blank line is encountered. A syntax error or end-of-stream will also halt processing.

Supply this routine with a reference to a filehandle glob; e.g., \*STDIN:

### Create a new header by parsing in STDIN:
$head->read(\*STDIN);

On success, the self object will be returned; on failure, a false value.

Note: in the MIME world, it is perfectly legal for a header to be empty, consisting of nothing but the terminating blank line. Thus, we can't just use the formula that "no tags equals error".

Warning: as of the time of this writing, Mail::Header::read did not flag either syntax errors or unexpected end-of-file conditions (an EOF before the terminating blank line). MIME::ParserBase takes this into account.

Normally, the new occurrence will be appended to the existing occurrences. However, if the optional INDEX argument is 0, then the new occurrence will be prepended. If you want to be explicit about appending, specify an INDEX of -1.

Warning: this method always adds new occurrences; it doesn't overwrite any existing occurrences... so if you just want to change the value of a field (creating it if necessary), then you probably don't want to use this method: consider using replace() instead.

This method has been deprecated. See "decode_headers" in MIME::Parser for the full reasons. If you absolutely must use it and don't like the warning, then provide a FORCE:

"I_NEED_TO_FIX_THIS"
Just shut up and do it. Not recommended.
Provided only for those who need to keep old scripts functioning.
"I_KNOW_WHAT_I_AM_DOING"
Just shut up and do it. Not recommended.
Provided for those who REALLY know what they are doing.

What this method does. For an example, let's consider a valid email header you might get:

Note: currently, the decodings are done without regard to the character set: thus, the Q-encoding =F8 is simply translated to the octet (hexadecimal F8), period. For piece-by-piece decoding of a given field, you want the array context of MIME::Words::decode_mimewords().

Warning: the CRLF+SPACE separator that splits up long encoded words into shorter sequences (see the Subject: example above) gets lost when the field is unfolded, and so decoding after unfolding causes a spurious space to be left in the field. THEREFORE: if you're going to decode, do so BEFORE unfolding!

This method returns the self object.

Thanks to Kent Boortz for providing the idea, and the baseline RFC-1522-decoding code.

Instance method, override. Print the header out to the given OUTSTREAM, or the currently-selected filehandle if none. The OUTSTREAM may be a filehandle, or any object that responds to a print() message.

The override actually lets you print to any object that responds to a print() method. This is vital for outputting MIME entities to scalars.

Also, it defaults to the currently-selected filehandle if none is given (not STDOUT!), so please supply a filehandle to prevent confusion.

All of the following methods extract information from the following fields:

Content-type
Content-transfer-encoding
Content-disposition

Be aware that they do not just return the raw contents of those fields, and in some cases they will fill in sensible (I hope) default values. Use get() or mime_attr() if you need to grab and process the raw field text.

Note: some of these methods are provided both as a convenience and for backwards-compatibility only, while others (like recommended_filename()) really do have to be in MIME::Head to work properly, since they look for their value in more than one field. However, if you know that a value is restricted to a single field, you should really use the Mail::Field interface to get it.

Instance method. Try real hard to determine the content type (e.g., "text/plain", "image/gif", "x-weird-type", which is returned in all-lowercase. "Real hard" means that if no content type could be found, the default (usually "text/plain") is returned. From RFC 2045 section 5.2:

Default RFC 822 messages without a MIME Content-Type header are
taken by this protocol to be plain text in the US-ASCII character
set, which can be explicitly specified as:
Content-type: text/plain; charset=us-ascii
This default is assumed if no Content-Type header field is specified.

Unless this is a part of a "multipart/digest", in which case "message/rfc822" is the default. Note that you can also set the default, but you shouldn't: normally only the MIME parser uses this feature.

Instance method. If this is a header for a multipart message, return the "encapsulation boundary" used to separate the parts. The boundary is returned exactly as given in the Content-type: field; that is, the leading double-hyphen (--) is not prepended.

Well, almost exactly... this passage from RFC 2046 dictates that we remove any trailing spaces:

If a boundary appears to end with white space, the white space
must be presumed to have been added by a gateway, and must be deleted.

Returns undef (not the empty string) if either the message is not multipart or if there is no specified boundary.

Instance method. Return the recommended external filename. This is used when extracting the data from the MIME stream. The filename is always returned as a string in Perl's internal format (the UTF8 flag may be on!)

I quote from Achim Bohnet, who gave feedback on v.1.9 (I think he's using the word "header" where I would use "field"; e.g., to refer to "Subject:", "Content-type:", etc.):

There is also IMHO no requirement [for] MIME::Heads to look
like [email] headers; so to speak, the MIME::Head [simply stores]
the attributes of a complex object, e.g.:
new MIME::Head type => "text/plain",
charset => ...,
disposition => ..., ... ;

We could even make it a superclass of MIME::Head: that way, MIME::Head would have to implement its interface, and allow itself to be initialized from a MIME::Attrs object.

However, when you read RFC 2045, you begin to see how much MIME information is organized by its presence in particular fields. I imagine that we'd begin to mirror the structure of RFC 2045 fields and subfields to such a degree that this might not give us a tremendous gain over just having MIME::Head.

Looking at a typical mail message header, it is sooooooo tempting to just store the fields as a hash of strings, one string per hash entry. Unfortunately, there's the little matter of the Received: field, which (unlike From:, To:, etc.) will often have multiple occurrences; e.g.:

The Received: field is used for tracing message routes, and although it's not generally used for anything other than human debugging, I didn't want to inconvenience anyone who actually wanted to get at that information.

I also didn't want to make this a special case; after all, who knows what other fields could have multiple occurrences in the future? So, clearly, multiple entries had to somehow be stored multiple times... and the different occurrences had to be retrievable.